Surface emitting semiconductor laser device

ABSTRACT

A surface emitting semiconductor laser device including a substrate and a layer structure formed thereon, the layer structure including an active layer, p-type and n-type distributed Bragg reflector (DBR) sandwiching therebetween the active layer, and first and second selectively-oxidized layers disposed within or in vicinities of the p-type and n-type DBRs, respectively, each of the p-type and n-type DBRs including a plurality of layer pairs each including a lower reflection layer and a higher reflection layer, each of the selectively-oxidized layers including a central Al x Ga 1-x As area (x≧0.98) and a peripheral oxide area formed by oxidizing an outer periphery of the central Al x Ga 1-x As area, the peripheral oxide area of the first selectively-oxidized layers has a width smaller than a width of the peripheral oxide area of the second selectively-oxidized layer. The surface emitting semiconductor laser device can be obtained having the operational voltage which can be maintained lower and the excellent single transverse mode.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to a surface emitting semiconductorlaser device with a current confinement oxide layer, and more inparticular to the a surface emitting semiconductor laser device with thecurrent confinement oxide layer having an excellent single transversemode.

[0003] (b) Description of the Related Art

[0004] As shown in FIG. 1, a conventional surface emitting semiconductorlaser device 10 has a layer structure, overlying a p-type GaAs substrate12, including a bottom distributed Bragg reflector (hereinafter referredto as “DBR”) 14 having a multiple-layer film including 20 to 30 pairs ofp-type Al_(y)Ga_(1-y)As/Al_(z)Ga_(1-z)As (y>z, y≦0.95), a currentconfinement structure including an AlAs layer 16 and an AlO_(x) layer 18formed by oxidizing the outer periphery of the AlAs layer 16, a p-typecladding layer 20, an active layer 22, an n-type cladding layer 24 and atop DBR 26 having a multiple-layer film including 20 to 30 pairs ofn-type Al_(y)Ga_(1-y)As/Al_(z)Ga_(1-z)As (y>z, y≦0.95).

[0005] The AlO_(x) current confinement layer 18 is formed by selectivelyoxidizing the Al in the AlAs layer 16 along the side walls of the airpost structure. The central part of the AlAs layer 16 remains unoxidizedto form a current injection path.

[0006] A ring-shaped n-side electrode 28 is formed on the top DBR 26,and a p-side electrode 30 is formed on the bottom surface of the p-typeGaAs substrate 12.

[0007] The operation of the surface emitting semiconductor laser device10 should be in the basic mode by stabilizing the transverse mode, i.e.in the single transverse mode, in order to use the laser device 10 as anoptical source.

[0008] The diameter of the active region (current injection region) 16surrounded by the AlO_(x) current confinement layer 18 should be 10 μmor less for achieving the single transverse mode.

[0009] Although a smaller diameter of 10 μm or less improves the singletransverse mode of the surface emitting semiconductor laser device tolower the threshold current thereof, problems arise such as increase ofthe operational voltage and deterioration of the temperaturecharacteristic due to the rise of an electric resistance caused by thereduction of the active region and due to the rise of the thermalresistance caused by the reduction of the thermal conductivity of theoxide layer.

[0010] Thus, in place of reducing the diameter of the active region tobe 10 μm or less, reduction of the aperture diameter of the n-sideelectrode or usage of a dielectric aperture is attempted to stabilizethe transverse mode.

[0011] However, in the above proposals, the apertures in theseelectrodes or the dielectric films formed by the photolithographic andetching process generate a deviation more than a submicron order betweenthe center of the active region and the center of the aperture.Accordingly, in reality, the stabilization of the transverse mode isdifficult.

[0012] Another method is reported in which a cladding layer is thickenedto obtain a single transverse mode (E. J. Ebeling et al., “HighPerformance VCSELs for Optical Data Links”, OECC 2000, pp.518-519,2000).

[0013] This method is effective at a lower output range wherein thesingle transverse mode uses 9 mA for the injection current at thehighest.

[0014] A further method is reported in which several pairs of mirrorsformed on a current confinement layer are oxidized to form apertures tomake a single transverse mode in a surface emitting semiconductor laserdevice formed on an n-type substrate (N. Nishiyama et el., “Multi-Oxidelayer Structure for Single-Mode Operation In Vertical-CavitySurface-Emitting Lasers”, IEEE Photonics technol. Lett., vol.12,pp.606-608).

[0015] The apertures in this method formed in a p-type semiconductormulti-layered film increase the resistance, and the aperture diametersare not decreased. The current flowing in the injection current regionin which the single transverse mode is stabilized is lower than 2 mA,and this method is effective also only for the lower output range.

[0016] As described above, realization of the surface emittingsemiconductor laser device is difficult which stably emits laser in thesingle transverse mode in the higher output range.

SUMMARY OF THE INVENTION

[0017] It is an object of the present invention to provide a surfaceemitting semiconductor laser device having a current confinement oxidelayer and is capable of stably operating in a single transverse mode.

[0018] In the present invention, a surface emitting semiconductor laserdevice is provided which includes a substrate and a layer structureformed thereon, the layer structure including an active layer, p-typeand n-type distributed Bragg reflector (DBR) sandwiching therebetweenthe active layer, and first and second selectively-oxidized layersdisposed within or in vicinities of the p-type and n-type DBRs,respectively, each of the p-type and n-type DBRs including a pluralityof layer pairs each including a lower reflection layer and a higherreflection layer, each of the selectively-oxidized layers including acentral Al_(x)Ga_(1-x)As area (x≧0.98) and a peripheral oxide areaformed by oxidizing an outer periphery of the central Al_(x)Ga_(1-x)Asarea, the peripheral oxide area of the first selectively-oxidized layershas a width smaller than a width of the peripheral oxide area of thesecond selectively-oxidized layer.

[0019] In accordance with the present invention, the surface emittingsemiconductor laser device has an excellent single transverse mode and alower operational voltage by using the oxide narrowing layers formedwithin or in the vicinity of the n-type semiconductor multi-layered filmand within or in the vicinity of the p-type semiconductor multi-layeredfilm as the optical confinement layer and the current confinement layer,respectively.

[0020] The above and other objects, features and advantages of thepresent invention will be more apparent from the following description.

BRIEF DESCRIPTION OF DRAWINGS

[0021]FIG. 1 is a sectional view showing a conventional surface emittingsemiconductor laser device.

[0022]FIG. 2 is a sectional view showing a surface emittingsemiconductor laser device in accordance with an Embodiment of thepresent invention.

[0023]FIGS. 3A and 3B show layer structures of compound semiconductorlayers in FIG. 2 constituting a bottom DBR and a top DBR, respectively.

[0024]FIG. 4 is a sectional view showing a surface emittingsemiconductor laser device in accordance with another Embodiment of thepresent invention.

[0025]FIGS. 5A and 5B show layer structures of compound semiconductorlayers in FIG. 4 constituting a bottom DBR and a top DBR, respectively.

PREFERRED EMBODIMENTS OF THE INVENTION

[0026] Then, the configuration of a semiconductor laser device inaccordance with embodiments of the present invention will be describedreferring to the annexed drawings.

Embodiment 1

[0027] A surface emitting semiconductor laser device 40 of theEmbodiment features an Al oxide layer acting as an optical confinementlayer.

[0028] As shown in FIG. 2, the surface emitting semiconductor laserdevice 40 has a layer structure, overlying a p-type (100) (or 10° off orless) GaAs substrate 42, including a p-type GaAs buffer layer 44, abottom DBR 46, a current confinement structure having a p-typeAl_(x)Ga_(1-x)As (x≧0.98) layer 48 and a first AlO_(x) layer 50 formedby selectively oxidizing an outer periphery of the Al_(x)Ga_(1-x)Aslayer 48, a bottom cladding layer 52, an active layer 54, a top claddinglayer 56, a top DBR 58 and an n-type GaAs contact layer 60.

[0029] As shown in FIG. 3A, the bottom DBR 46 is configured as asemiconductor multi-layer film including 20 to 40 pairs of a p-typeAl_(y)Ga_(1-y)As layer 46 a acting as a lower reflection layer and ap-type Al_(z)Ga_(1-z)As layer 46 b acting as a higher reflection layer(y>z, y≦0.95).

[0030] As shown in FIG. 3B, the top DBR 58 is configured as asemiconductor multi-layer film including 15 to 30 pairs of an n-typeAl_(y)Ga_(1-y)As layer 58 a acting as a lower reflection layer and ann-type Al_(z)Ga_(1-z)As layer 58 b acting as a higher reflection layer(y>z, z≦0.95).

[0031] In place of one of the plenty of the n-type Al_(y)Ga_(1-y)Aslayers 58 a in the top DBR 58, an n-type Al_(x)Ga_(1-x)As (x≧0.98) layer62 is formed and the outer periphery of the Al_(x)Ga_(1-x)As layer isselectively oxidized to form a second AlO_(x) layer 64.

[0032] The n-type GaAs contact layer 60, the top DBR 58, the n-typeAl_(x)Ga_(1-x)As layer 62 and the AlO_(x) layer 64, the top claddinglayer 56, the active layer 54, the bottom cladding layer 52, the p-typeAl_(x)Ga_(1-x)As layer 48, the current confinement structure of thefirst AlO_(x) layer 50 and the upper part of the bottom DBR 46 areconfigured as a mesa post structure.

[0033] The first AlO_(x) layer 50 is formed by selectively oxidizing theAl in the p-type Al_(x)Ga_(1-x)As layer 48 along the side walls of themesa post structure to generate a region no current flows.

[0034] The central region of the p-type Al_(x)Ga_(1-x)As layer 48remains unoxidized to form a current injection path having a diameter of15 μm (hereinafter referred to as “second aperture 65”). Thereby, theregion no current flows is restricted and the diameter of the lightemitting region is determined.

[0035] The second AlO_(x) layer 64 is formed by selectively oxidizingthe Al in the n-type Al_(x)Ga_(1-x)As layer 62 along the side walls ofthe mesa post structure.

[0036] The central region of the n-type Al_(x)Ga_(1-x)As layer 62remains unoxidized to form the n-type Al_(x)Ga_(1-x)As layer 62 having adiameter of 10 μm (hereinafter referred to as “first aperture 67”).

[0037] The bottom cladding layer 52, the active layer 54 and the topcladding layer 56 form a resonator of the surface emitting semiconductorlaser device 40.

[0038] A ring-shaped n-side electrode 66 is formed on the n-type GaAscontact layer 60, and a p-side electrode 68 is formed on the bottomsurface of the p-type GaAs substrate 42.

[0039] The refractivity of the second AlO_(x) layer is up to 1.7, andthe refractivity of the first aperture 67 of the n-type Al_(x)Ga_(1-x)Aslayer 62 where the central part is unoxidized is up to 3. Accordingly,the rays propagate through the first aperture (n-type Al_(x)Ga_(1-x)Aslayer) 62.

[0040] A similar phenomenon takes place in the first AlO_(x) layer 50.

[0041] Then, the diameters of the second aperture 65 and of the firstaperture 67 are defined as “D” and “d”, respectively.

[0042] The diameter of the light emitting region is restricted by thecurrent flow limited by the second aperture having the diameter “D”,thereby reducing the threshold current. While the transverse mode iscontrolled by the refractivity difference between the first AlO_(x)layer 50 and the second aperture 65, the single transverse mode can beobtained only when the diameter “D” is about 5 μm or less because therefractivity difference between the oxidized layer and the unoxidizedlayer is larger.

[0043] However, if the diameter “D” is made to be up to 5 μm, thethermal resistance increases to worsen the temperature characteristicsand the electric resistance increases to raise the operational voltage.If the diameter “D” is made to be about 15 μm to maintain the loweroperational voltage, the multiple transverse modes are generated.

[0044] On the contrary, in the present Embodiment, the higher order modeof the transverse mode is cut off to realize the single transverse modeby making the diameter “D” of the second aperture 65 to be 15 μm for thecurrent confinement and further making the diameter “d” of the firstaperture 67 formed in the bottom DBR 46 to be 10 μm.

[0045] In other words, the oxide width of the first AlO_(x) layer 50surrounding the second aperture 65 is smaller than the second AlO_(x)layer 64 surrounding the first aperture 67 in the n-type bottom DBR 46.

[0046] If the diameter “d” of the first aperture is reduced, theelectric resistance does not increase significantly and the operationalvoltage does not increase because of the n-type.

[0047] The current confinement structure formed by the Al_(x)Ga_(1-x)Aslayer 48 and the first AlO_(x) layer 50 in the present Embodiment isformed between the bottom DBR 46 and the bottom cladding layer 52.However, the current confinement structure may be formed in the bottomDBR 46.

[0048] The optical confinement structure formed by the n-typeAl_(x)Ga_(1-x)As layer 62 and the second AlO_(x) layer 64 is formed inplace of one of the plenty of the n-type Al_(y)Ga_(1-y)As layers 58 a inthe top DBR 58. However, the n-type Al_(x)Ga_(1-x)As layer may be formedbetween the top cladding layer 56 and the top DBR 58 and the secondAlO_(x) layer 64 is formed by selectively oxidizing the outer peripheryof the n-type Al_(x)Ga_(1-x)As layer.

Embodiment 2

[0049] As shown in FIG. 4, a surface emitting semiconductor laser device70 of the Embodiment, another example of the present invention, isformed on an n-type GaAs substrate and has a basic configuration inwhich the bottom DBR and the top DBR of the surface emittingsemiconductor laser device in Embodiment 1 are replaced with each other.

[0050] That is, as shown in FIG. 4, the surface emitting semiconductorlaser device 70 has a layer structure, overlying an n-type (100) (or 10°off or less) GaAs substrate 72, including an n-type GaAs buffer layer74, a bottom DBR 76, a bottom cladding layer 78 formed by an n-typeAl_(x)Ga_(1-x)As (x≧0.98) layer, an active layer 80, a top claddinglayer 82 formed by an n-type Al_(x)Ga_(1-x)As (x≧0.98) layer, a currentconfinement structure having a p-type Al_(x)Ga_(1-x)As (x≧0.98) layer 84and a first AlO_(x) layer 86 formed by selectively oxidizing an outerperiphery of the Al_(x)Ga_(1-x)As layer 84, a top DBR 88 and a p-typeGaAs contact layer 90.

[0051] As shown in FIG. 5A, the bottom DBR 76 is configured as asemiconductor multilayer film including 15 to 30 pairs of an n-typeAl_(y)Ga_(1-y)As layer 76 a acting as a lower reflection layer and ann-type Al_(z)Ga_(1-z)As layer 76 b acting as a higher reflection layer(y>z, y≦0.95).

[0052] As shown in FIG. 5A, in place of one of the n-typeAl_(y)Ga_(1-y)As layers 76 a constituting the bottom DBR 76 in the mesapost, an n-type Al_(x)Ga_(1-x)As (x≧0.98) layer 92 is formed and theouter periphery of the Al_(x)Ga_(1-x)As layer is selectively oxidized toform a second AlO_(x) layer 94.

[0053] As shown in FIG. 5B, the top DBR 88 is configured as asemiconductor multi-layer film including 20 to 40 pairs of an n-typeAl_(y)Ga_(1-y)As layer 88 a acting as a lower reflection layer and ann-type Al_(z)Ga_(1-z)As layer 88 b acting as a higher reflection layer(y>z, z≦0.95).

[0054] The p-type GaAs contact layer 90, the top DBR 88, the currentconfinement structure formed by the p-type Al_(x)Ga_(1-x)As layer 84 andthe AlO_(x) layer 86, the top cladding layer 82, the active layer 80,the bottom cladding layer 78, the n-type Al_(x)Ga_(1-x)As layer 92 andthe second AlO_(x) layer 94 and the upper part of the bottom DBR 76 areconfigured as a mesa post structure.

[0055] The first AlO_(x) layer 86 is formed by selectively oxidizing theAl in the p-type Al_(x)Ga_(1-x)As layer 84 along the side walls of themesa post structure to generate a region no current flows.

[0056] The central region of the p-type Al_(x)Ga_(1-x)As layer 84remains unoxidized to form a current injection path having a diameter of15 μm (hereinafter referred to as “second aperture 96”). Thereby, theregion no current flows is restricted and the diameter of the lightemitting region is determined.

[0057] The second AlO_(x) layer 94 is formed by selectively oxidizingthe Al in the n-type Al_(x)Ga_(1-x)As layer 92 along the side walls ofthe mesa post structure.

[0058] The central region of the n-type Al_(x)Ga_(1-x)As layer 92remains unoxidized to form the n-type Al_(x)Ga_(1-x)As layer 92 having adiameter of 10 μm (hereinafter referred to as “first aperture 98”).

[0059] The bottom cladding layer 78, the active layer 80 and the topcladding layer 82 form a resonator of the surface emitting semiconductorlaser device 70.

[0060] A ring-shaped p-side electrode 100 is formed on the p-type GaAscontact layer 90, and an n-side electrode 102 is formed on the bottomsurface of the n-type GaAs substrate 72.

[0061] The refractivity of the second AlO_(x) layer 94 is up to 1.7, andthe refractivity of the first aperture 98 of the n-type Al_(x)Ga_(1-x)Aslayer 92 where the central part is unoxidized is up to 3. Accordingly,the rays propagate through the unoxidized first aperture (n-typeAl_(x)Ga_(1-x)As layer) 98 in the center of the mesa post.

[0062] A similar phenomenon takes place in the first AlO_(x) layer 86.

[0063] Then, the diameters of the second aperture 96 and of the firstaperture 98 are defined as “D” and “d”, respectively.

[0064] The diameter of the light emitting region is restricted by thecurrent flow limited by the second aperture 96 having the diameter “D”,thereby reducing the threshold current. While the transverse mode iscontrolled by the refractivity difference between the first AlO_(x)layer 86 and the second aperture 96, the single transverse mode can beobtained only when the diameter “D” is about 5 μm or less because therefractivity difference between the oxidized layer and the unoxidizedlayer is larger.

[0065] However, if the diameter “D” is made to be up to 5 μm, thethermal resistance increases to worsen the temperature characteristicsand the electric resistance increases to raise the operational voltage.If the diameter “D” is made to be about 15 μm to maintain the loweroperational voltage, the multiple transverse modes are generated.

[0066] On the contrary, in the present Embodiment, the higher order modeof the transverse mode is cut off to realize the single transverse modeby making the diameter “D” of the second aperture 96 to be 15 μm for thecurrent confinement and further making the diameter “d” of the firstaperture 98 formed in the bottom DBR 76 to be 10 μm.

[0067] In other words, the oxide width of the first AlO_(x) layer 86surrounding the second aperture 96 is smaller than the second AlO_(x)layer 94 surrounding the first aperture 98 in the n-type bottom DBR 76.

[0068] If the diameter “d” of the first aperture is reduced, theelectric resistance does not increase significantly and the operationalvoltage does not increase because of the n-type.

[0069] The current confinement structure formed by the p-typeAl_(x)Ga_(1-x)As layer 84 and the first AlO_(x) layer 86 in the presentEmbodiment is formed between the top cladding layer 80 and the top DBR82. However, the current confinement structure may be formed in thebottom DBR 80. In other words, the p-type Al_(x)Ga_(1-x)As layer isformed in place of one of the p-type Al_(y)Ga_(1-y)As layers 80 a in thebottom DBR 80, and the outer periphery of the p-type Al_(x)Ga_(1-x)Aslayer is oxidized to form the first AlO_(x) layer.

[0070] The optical confinement structure formed by the n-typeAl_(x)Ga_(1-x)As layer 92 and the second AlO_(x) layer 94 is formed inplace of one of the plenty of the n-type Al_(y)Ga_(1-y)As layers 58 a inthe bottom DBR 76. However, the n-type Al_(x)Ga_(1-x)As layer may beformed between the bottom DBR 75 and the bottom cladding layer 78, andthe second AlO_(x) layer may be formed by selectively oxidizing theouter periphery of the n-type Al_(x)Ga_(1-x)As layer.

[0071] Since the above embodiment is described only for examples, thepresent invention is not limited to the above embodiment and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention.

What is claimed is:
 1. A surface emitting semiconductor laser devicecomprising a substrate and a layer structure formed thereon, the layerstructure including an active layer, p-type and n-type distributed Braggreflector (DBR) sandwiching therebetween the active layer, and first andsecond selectively-oxidized layers disposed within or in vicinities ofthe p-type and n-type DBRs, respectively, each of the p-type and n-typeDBRs including a plurality of layer, pairs each including a lowerreflection layer and a higher reflection layer, each of theselectively-oxidized layers including a central Al_(x)Ga_(1-x)As area(x≧0.98) and a peripheral oxide area formed by oxidizing an outerperiphery of the central Al_(x)Ga_(1-x)As area, the peripheral oxidearea of the first selectively-oxidized layers has a width smaller than awidth of the peripheral oxide area of the second selectively-oxidizedlayer.
 2. The surface emitting semiconductor laser device as defined inclaim 1, wherein the selectively-oxidized layer replaces the lowerreflection layer in each of the p-type and n-type DBRs.
 3. The surfaceemitting semiconductor laser device as defined in claim 1, the firstselectively-oxidized layer resides within the p-type DBR and functionsas a current confinement layer, and the second selectively oxidizedlayer resides within the n-type DBR and functions as an opticalconfinement layer.
 4. The surface emitting semiconductor laser device asdefined in claim 1, wherein the Al_(x)Ga_(1-x)As area of secondselectively-oxidized layer has a diameter of 10 μm or less.
 5. Thesurface emitting semiconductor laser device as defined in claim 1,wherein the lower reflection layer and the higher reflection layer areAl_(y)Ga_(1-y)As layer and Al_(z)Ga_(1-z)As layer, respectively, given yand z are such that y>z and 0≦y≦0.95.